1. Field of the Invention
The present invention relates to an exposure control device for a camera in which transient responsive characteristics of a light measuring circuit are improved.
2. Description of the Prior Art
A conventional light measuring circuit employing a photo-electric conversion element of a photo-voltaic type as a light receiving element has an operational amplifier with a high input impedance utilizing an FET at its input stage connected to said light receiving element. Unnecessary charging is instantaneously performed by stray capacitances since capacitance exists; at a gate of the FET when a power is supplied from a power source, junction capacitance exits in the light receiving element and there exists capacitance in the printed circuit network, etc. A certain length of time is necessary to discharge such unnecessary charge. Thus, the light measuring circuit is difficult to stabilize. This disadvantage is serious, in particular, when the amount of photo-current is small when a very weak light is sensed.
Furthermore, since a photo-current to a logarithmic conversion diode inserted in a negative feedback circuit is cut off in a conventional light measuring circuit in a blacked out state, negative feedback will not work on the operational amplifier with a high input impedance, and an output of the amplifier will become either a high level or a low level signal depending on a direction of the diode. That is, the amplifier is latched. Therefore, some length of time is necessary to release said latch of the amplifier before the next photograph is to be taken and follower characteristics of the light measuring circuit against an amount of incident light become poor.
These disadvantages constitute drawbacks especially when light beams reflected from a film surface are sensed to effect control over an exposure time. That is, the light are beams reflected from a film surface to the light receiving element as a shutter is released. Also, in a camera in which light beams reflected from a film surface are sensed to effect exposure control, a closing signal has to be provided by a shutter control electro-magnet before the light beams reflected from a film surface have sufficiently impinged when a high speed shutter control is performed. Thus, an accurate exposure time control could not have been made.
An object of the present invention is to eliminate said disadvantages, and is to provide an exposure control device for a camera which is made so that a first light receiving element that senses light beams reflected from a film surface and effects an exposure control is connected between input terminals of a light measuring operational amplifier while a second light receiving element that senses light beams from an object before a photo-taking and make a preliminary display of an exposure is connected in parallel with the first light receiving element, so that transient responsive characteristics of a light measuring circuit at the time a photograph is taken are improved.
Another object of the present invention is to provide an automatic exposure control device for a camera in which an accurate exposure control is effected based on a signal synthesized from an output of the first light receiving element that senses light beams reflected from a film surface as a shutter is opened and from the second light receiving element which stops sensing light beams from an object as a shutter is opened.
Further, another object of the present invention is to provide an exposure control device for a camera in which an appropriate amount of biasing light beams is made to impinge, at the time a photograph is to be taken, into a light receiving element which senses light beams from an object before a photograph is taken in order to make a preliminary display of an exposure.